Typical examples of fluid jet methods for discharging a fluid such as ink onto an object (recording medium) are a piezo type and a thermal type that are commercially utilized in ink jet printers. In addition to them, there is an electrostatic suction type which is arranged as follows: the fluid to be discharged have electrical conductivity, and an electric field is applied to the conductive fluid so that the fluid is discharged through a nozzle.
Such an electrostatic suction type fluid discharge device is disclosed in, for example, Japanese Examined Patent Publication 36-13768 (published on Aug. 18, 1961) and Japanese Laid-Open Patent Application No. 2001-88306 (published on Apr. 3, 2001).
Also, Japanese Laid-Open Patent Application No. 2000-127410 (published on May 9, 2000) discloses a device arranged such that a nozzle hole is slit-shaped and a protruding needle electrode is formed in the nozzle hole, and an ink including fine particles is discharged using the needle electrode. Japanese Laid-Open Patent Application No. 8-238774 (published on Sep. 17, 1996) discloses a device in which an electrode for applying a voltage is provided in an ink container from which an ink is supplied to a nozzle hole.
The following describes a fluid discharge model in a conventional electrostatic suction type fluid discharge device.
Design factors of electrostatic suction type fluid discharge devices, especially of on-demand electrostatic suction type fluid discharge devices are, conductivity of an ink fluid (e.g. specific resistance of 106 to 1011 Ωcm), surface tension (e.g. 0.020 to 0.040N/m), viscosity (e.g. 0.011 to 0.015 Pa·s), and applied voltage (electric field). As to the applied voltage, it has been considered that the voltage applied to the nozzle and the distance between the nozzle and an opposing electrode are particularly important.
The electrostatic suction type fluid discharge devices utilize electrofluid instability, as shown in FIG. 32. Placing a conductive fluid in a uniform electric field, an electrostatic force exerted on the surface of the conductive fluid causes the surface to be instable, thereby precipitating the development of a thread (electrostatic thread-like phenomenon). The electric field on this occasion is defined as E0 which is generated when a voltage V is applied between a nozzle and an opposing electrode. The distance between a nozzle hole at the tip of the nozzle and the opposing electrode is defined as h. A development wavelength λC in the aforesaid case can be physically figured out (see, e.g. The Institute of Image Electronics Engineers of Japan, Vol. 17, No. 4, 1988, pp. 185-193), and the developing wavelength λC is represented by the following equation.
                              λ          c                =                                            2              ⁢              πγ                                      ɛ              0                                ⁢                      E            0                          -              2                                                          (        1        )            
In the equation, γ is surface tension (N/m), ∈0 is dielectric constant (F/m) in a vacuum, and E0 is electric field strength (V/m). If the nozzle diameter d(m) is shorter than λC, the development does not occur. That is, the condition of the discharging is defined as follows.
                              d          >                                    λ              c                        2                          =                  πγ                                    ɛ              0                        ⁢                          E              0              2                                                          (        2        )            
Provided that E0 is an electric field strength (V/m) on the assumption that a parallel flat plate is adopted, h(m) is the distance between the nozzle and opposing electrode, and V0 is a voltage applied to the nozzle, the following equation is given:
                              E          0                =                              V            0                    h                                    (        3        )            
Therefore, the following formula is also given:
                    d        >                              πγ            ⁢                                                  ⁢                          h              2                                                          ɛ              0                        ⁢                          V              0              2                                                          (        4        )            
The fluid discharge devices have typically been required to reduce the diameter of the nozzle through which ink is discharged, in order to form finer dots and lines.
However, in the currently-used piezo or thermal fluid discharge devices, it is difficult to reduce the nozzle diameter and discharge, for example, a very small amount of fluid less than 1 pl. This is because, the smaller the nozzle for discharging a fluid is, the more the pressure required for the discharge increases.
In addition to the above, in the aforesaid fluid discharge devices, the miniaturization of droplets contradicts the improvement of precision, and hence it has been difficult to realize both of these improvements at the same time. The reason of this will be described below.
Kinetic energy imparted to the droplet discharged from the nozzle is in proportion to the cube of the diameter of the droplet. Therefore, the minute droplets discharged in a case where the nozzle is miniaturized cannot attain the kinetic energy sufficient to resist the air resistance at the time of the discharge, and the droplets are disturbed by accumulated air or the like. For this reason, it is not possible to expect precise landing of the droplets. Moreover, since the effect of the surface tension increases as the size of the droplets decreases, the vapor pressure of the droplets increases and an amount of evaporation increases. As a result, a great amount of each minute droplet gets lost while flying, and each droplet hardly retains its shape at the time of landing.
In addition to the above, according to the aforesaid fluid discharge model of the conventional electrostatic suction type fluid discharge devices, the reduction of the nozzle diameter demands the increase in the electric field strength, which is necessary for the discharge, as the above-described equation (2) shows. The electric field strength is, as shown in the equation (3), determined by the voltage (drive voltage) V0 applied to the nozzle and the distance h between the nozzle and opposing electrode. Therefore, the reduction of the nozzle diameter results in the increase in the drive voltage.
The drive voltage in the conventional electrostatic suction type fluid discharge devices is very high (not less than 1000V). It is therefore difficult to achieve the reduction in size and the density growth, in consideration of leaks and interferences between the nozzles. Also, the problem becomes serious as the nozzle diameter is further reduced. A power semiconductor with a high voltage of not less than 1000V is typically expensive and does not excel in frequency responsiveness.
In the Japanese Examined Patent Publication 36-13768, the nozzle diameter is 0.127 mm. The range of the nozzle diameter in Japanese Laid-Open Patent Application No. 2001-88306 is 50 to 2000 μm, more preferably 100 to 1000 μm.
As to the nozzle diameter, the development wavelength λC is worked out as follows, if typical operating conditions of the conventional electrostatic suction type fluid discharge are applied: the development wavelength λC is about 140 μm where the surface tension is 0.020N/m and the electric field strength is 107V/m in the aforesaid equation (1). Consequently, the limit nozzle diameter is 70 μm. It has therefore been considered that, in a case where the nozzle diameter is not more than about 70 μm in the aforesaid conditions, the ink development does not occur even if the field strength is high (107V/m), unless a countermeasure such as forcible formation of meniscus by the application of a back pressure is carried out. In short, it has been considered that the miniaturization of the nozzle is not compatible with the reduction in the drive voltage.
As described above, in the conventional fluid discharge devices, the miniaturization of the nozzle contradicts with the improvement of precision, and it has been difficult to achieve both of these improvements. In particular, regarding the electrostatic suction type fluid discharge devices, it has been considered that the miniaturization of the nozzle contradicts with the reduction in the drive voltage.